Officials in China say 91 people are missing after a landslide in the southern city of Shenzhen Sunday morning.

The Ministry of Land and Resources said heavy rain soaked a 100-meter tall pile of construction waste and dirt. The pile was too big and too tall, which caused “instability and collapse.”

Witnesses say huge waves of mud swept into the area. Thirty-three buildings were destroyed or damaged. They include 14 factories, two office buildings and three buildings where workers lived. More than 1,500 emergency workers are searching for survivors.

The landslide also caused an explosion at a natural gas pipeline in the area.

The country’s official news agency Xinhua says 59 men and 32 women are missing. About 900 people were removed from the area before the landslide happened.